This application claims the priority of Japanese Patent Applications No. 2001-102276 filed on Mar. 30, 2001 which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to the structure of a light source section of an electronic endoscope apparatus that supplies a subject with a light using an AC lighting lamp that discharges and lights when provided with an alternating current.
2. Description of the Related Art
With an electronic endoscope apparatus, a light source light is guided from a light source device or the like to a tip portion of a scope via a light guide. Then, an image pickup device such as a CCD (Charge Coupled Device) is used to pickup an image of a subject on the basis of light irradiation from the tip portion so that the subject is displayed on a monitor. In recent years, it has been proposed that an AC lighting lamp, also called a xe2x80x9chigh-luminance discharge lampxe2x80x9d or the like, be used as a light source for the light source device.
That is, compared to halogen lamps, that is, filament lamps that light when tungsten wires contained therein are heated, or other similar lamps, xenon lamps and metal halide lamps, both of which are discharge lamps, are advantageous in that they efficiently emit a light and withstand long use. Further, endoscopes have a thin light guide corresponding to a scope having a reduced diameter, so that discharge lamps having a smaller luminous bright spot allow a light to impinge more efficiently on the thin light guide.
FIG. 11 shows a configuration of an AC lighting lamp (discharge lamp). A lamp 1 in FIG. 11 is composed of a first electrode (cathode) 2 having a sharply angled tip, a second electrode (anode) 3 having a similarly shaped tip, and a reflector 4. If the reflector 4 has a reflecting surface shaped like, for example, an ellipse K (which has a center O1), a bright spot g1 of the sharply-angled tip of the first electrode 2 is arranged at a first focus F1 of the ellipse K, with an entrance facet of a light guide 5 arranged at a second focus F2. With this AC lighting lamp 1, lights emitted from the bright spot g1 are condensed at the second focus F2 by the reflector 4, and the condensed light is guided to the tip portion of the scope via the light guide 5.
However, with an electronic endoscope apparatus using the AC lighting lamp 1, a current applied between the electrodes alternately changes its direction (positive or negative), and the two bright spots g1 and g2 are alternately lighted. If the lighting frequency is lower than the frequency (cycle) at which an image pickup device such as a CCD accumulates charges, an exposure difference may occur between fields to cause flickers.
FIGS. 12(A) and 12(B) show an AC current applied to the AC lighting lamp 1 and a lamp emission state. As shown in FIG. 12(A), the lamp 1 alternates between application of a xe2x88x92e voltage to the first electrode (cathode) 2 and application of a +e voltage to the second electrode (anode) 3. As shown in FIG. 12(B), lighting q (lighting frequency fq) is intermittently carried out at the bright spot g1 in FIG. 11.
FIGS. 13(A) to 13(D) show the relationship between the accumulation of charges in the CCD and the lighting of the lamp. If the frequency fq of the lighting q of the lamp 1 is higher than the frequency (vertical synchronizing signal frequency) fc of charge accumulation c (fq greater than fc), two lightings q (shaded portions) are uniformly carried out within the time of charge accumulation c as shown by the odd and even fields in FIGS. 13(A) and 13(B). However, If the frequency fq is lower than the frequency fc of charge accumulation c (fqxe2x89xa6fc), then as shown by the odd and even fields in FIGS. 13(C) and 13(D), the lightings (emissions) q are not uniformly carried out within the time of charge accumulation c as shown by shaded portions. As a result, the amount of exposure varies between the fields, and when the odd fields in FIG. 13(C) and the even fields in FIG. 13(D) are superimposed on each other so that the former alternates with the latter, flickers may occur on the screen.
The present invention is achieved in view of the above problems, and it is an object thereof to provide an electronic endoscope apparatus provided with an AC lighting light source that allows images to be picked up with a uniform amount of emission even if the lighting frequency is lower than the frequency of charge accumulation within the corresponding time, thereby preventing the occurrence of flickers.
To attain the above object, the present invention is characterized by comprising an AC lighting light source having condensing means, a light guide that guides a light from the light source to a tip portion of a scope, and an image pickup element that picks up an image of a subject irradiated with a light via the light guide, wherein an entrance facet of the light guide is arranged between two condensing points and at a position that is not close to either of the condensing points so that lights from a bright spot at the tip of each of two electrodes of the AC lighting light source are condensed at the corresponding condensing point by the condensing means.
If as the condensing means for the AC lighting light source, a reflector is arranged opposite the light guide relative to the light source, when an arc gap length between the two electrodes is defined as xcex94l, then the entrance facet of the light guide may be arranged close to a position located at a distance +(xcex94l/2) or xe2x88x92(xcex94l/2) from the condensing point formed on the basis of the shape of the reflector.
If as the condensing means for the AC lighting light source, a reflector is arranged opposite the light guide relative to the light source and a condensing lens is arranged on the same side as that of the light guide relative to the light source, when the arc gap length between the two electrodes is defined as xcex94l, the focal distance of the condensing lens is defined as f, and the distance from the front focal position of the condensing lens to the focal position of the reflector is defined as x, then the entrance facet of the light guide is preferably arranged close to a position located at the distance indicated by Equation 1, shown below, from the condensing point formed by the reflector and condensing lens.                                           +                                                                                f                    2                                    ·                  Δ                                ⁢                                  xe2x80x83                                ⁢                l                                            2                ⁢                                  x                  ⁡                                      (                                          x                      +                                              Δ                        ⁢                                                  xe2x80x83                                                ⁢                        l                                                              )                                                                                ⁢                      xe2x80x83                    ⁢          or                ⁢                  xe2x80x83                -                                                            f                2                            ·              Δ                        ⁢                          xe2x80x83                        ⁢            l                                2            ⁢                          x              ⁡                              (                                  x                  +                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    l                                                  )                                                                        [Equation 1]            
In the AC lighting light source, a large amount of emission from the two luminous bright spots present at the tips of the respective electrodes is not obtained between these bright spots. However, discharge emission reciprocating between the two electrodes yields emission with a reduced temporal variation in luminance. The present invention utilizes such emission with a reduced temporal variation in luminance, and the entrance facet of the light guide is arranged between the two condensing points so that lights from one of the two bright spots are condensed at the corresponding condensing point by the reflector or condensing lens.
Preferably, if only the reflector is provided, the entrance facet of the light guide is arranged close to the position located at the distance xc2x1(xcex94l/2) from the corresponding condensing point. If both reflector and condensing lens are provided, the entrance facet is arranged close to the position located at the distance xc2x1{f2xc2x7xcex94l/[2x(x+xcex94l)]} from the corresponding condensing point. This provides the light guide with a light having a reduced temporal variation in luminance, and affords a substantially uniform amount of emission (amount of exposure) even if the lighting frequency is lower than the frequency of charge accumulation.